Thermal ablation techniques are an excellent alternative to major surgery, which can pose a risk even with the most experienced surgeon. These techniques are minimally invasive, requiring only needles (radiofrequency (RF), cryotherapy and microwave ablation) or a non-invasive heat source such as by using ultrasound, e.g., high-intensity focused ultrasound (HIFU). In most of the procedures, the cancerous tissue is heated to above 60° Celsius (C) and subject to necrosis.
Radiofrequency ablation (RFA) is currently the only FDA approved minimally invasive heating therapy in the United States. RF ablation uses a probe with an active electrode tip through which a 460-500 kilohertz (KHz) alternating current is conducted. The current propagates through the body to the grounding pads placed either on the back or the thigh of the patient. The current causes ionic agitation and frictional heating. Heat is then dissipated through thermal conduction to ablate the tumor. RFA is frequently used to treat liver cancer. There are about 500,000 new cases of metastatic liver cancer in the western world and about 1 million new cases for primary liver cancer worldwide (83% of which are in developing countries). RFA and microwave ablation therapies are also gaining tremendous popularity in China due to the large number of liver cancers reported (e.g., 433,000 new cases in 2009 alone). Current treatment protocols use the simplistic spherical ablation volume predicted from the device manufacturers' specifications. The actual treatment volumes greatly deviate from the prediction, resulting in large recurrence rates (approx. 35%).
RF ablation is typically performed under ultrasound, computed tomography (CT) or magnetic resonance imaging (MRI) guidance. Follow up is done with a CT scan or MRI within a month to assess effectiveness of ablation and again at 3 month intervals along with tumor markers to detect residual disease or recurrence. One common reason for the high recurrence rates is the inability to monitor and control ablation size to adequately kill the tumor cells. Real-time feedback is accordingly provided to the clinician by means of a temperature map of the ablated zone. This can currently be achieved with reasonable accuracy with MR based temperature imaging. However, MRI is expensive and may not be readily available. Ultrasound is another modality that is commonly used for image guidance during placement of the needle. Due to its ease of use and availability it is a preferred method for monitoring the lesions. However, the only way it is currently used for monitoring treatment is by visualizing the hyperechoic lesions on a B-mode image. Low contrast exists between normal and ablated tissue. Visual artifacts arise from gas bubbles. Thus, the visualization currently afforded by ultrasound is only approximate and not a good indicator of the treatment efficacy. Also reliance on gas bubbles for echogenicity encounters the problem that bubble formation mainly occurs at temperatures elevated above those needed for the ablation, potentially resulting in unnecessary cell damage and prolongation of the procedure.
Another proposed ultrasound technique for ablation monitoring is ultrasound thermometry. Ultrasound thermometry can potentially enable mapping the temperature distribution during thermal therapies in 3D spatial and temporal dimensions. Through the concept of thermal dose (derived from the time history of temperature rise), the extent of the ablation zone can be determined over the entire volume. Hence, ultrasound thermometry provides significant advantages over temperature measurements obtained from a single or a few thermocouples that provide only a sparse sampling of the ablation zone. The underlying principle of ultrasound thermometry is that the speed of sound in the tissue changes as a function of temperature which manifests as apparent shifts (displacement) in ultrasound echoes. The resulting temperature induced strain (derived by differentiating the displacement along the direction of the ultrasound beam) is nominally proportional to the temperature rise in the range up to 50° C. The proportionality constant (thermal strain to temperature coefficient) is typically estimated through a calibration performed in a water bath wherein a known temperature rise that produces the corresponding thermal strain is noted. One such study discloses calibration curves for different body tissue types. Varghese, T., Daniels, M. J., “Real-time calibration of temperature estimates during radiofrequency ablation”, Ultrasonic Imaging, 26(3):185-200 (2004) (hereinafter “Varghese”). The curves, which each relate temperature rise to thermal strain, are each seen to be essentially linear over a hypothermia temperature range which extends up to 50° C. Accordingly, a proportionality constant can be derived for each tissue type. U.S. Patent Publication No. 2013/0204240 to McCarthy discloses an integrated catheter tip (ICT) that includes a thermocouple. The ICT is used for hyperthermia therapy. Readings from the thermocouple are used to measure temperature adjacent to the ICT. A radiometer is also used in the measurement, because heating is caused by microwave energy and because a more complete picture of the temperatures in the treatment region is desired.